1. Technical Field
This invention relates to portable telephones and, more particularly, to portable telephones employing packetized wireless transmissions.
2. Description of the Prior Art
Recent rulings promulgated by the Federal Communication Commission (FCC) for the use of spread spectrum systems, including a frequency hopping system, now allow for increased spectral utilization by portable telephones. Frequency hopping systems spread their energy by changing, or hopping, the center frequency of the transmission many times a second in accordance with a pseudo randomly generated list of communication channels. The result is a significantly higher signal to noise ratio than may be achieved by conventional techniques such as amplitude modulation that uses no bandwidth spreading.
Digital portable telephones are now being designed for use in spread spectrum systems, including frequency hopping systems. One such digital portable telephone is disclosed in U.S. patent application Ser. No. 07/779,754 filed in the name of M. E. Gillis et al. on Oct. 21, 1991. This digital portable telephone utilizes time division duplexing for communicating between a base unit and its associated handset unit. In the operation of such a portable telephone, the base unit and the handset unit communicate with each other by alternately transmitting and receiving a signal on the same channel. Thus, while the base unit is transmitting on a channel, the handset unit is configured for receiving the signal from the base unit over that same channel. Similarly, while the handset unit is transmitting on the channel, the base unit is configured for receiving the signal from the handset unit. Due to the packetizing handling of voice data in these digital portable telephones, delays which result in echoes discernible by the user of the handset unit may be generated at certain packetizing rates of the voice data.
Some propagation delay of the voice signal is normal and is present in all portable telephones. Presently available analog portable telephones, which employ analog technology in communicating between a base unit and a handset unit, delay the voice signal by propagation path delay and any analog circuit delays. These delays are at most several microseconds in length and are, therefore, not noticeable by a user of a handset unit in an analog portable telephone as his or her speech signal is returned to the handset unit and heard as sidetone. These delays are similarly associated with a digital portable telephone. With a digital portable telephone which also employs time division duplexing, however, there can be a significant delay between the time that the handset unit user speaks and the time that his or her speech signal is reflected back to the handset unit. This produces a noticeable and possibly annoying sidetone echo to the user of the handset unit while he or she is speaking.